crane/docs/faq/workspace-not-at-source-root.md

Cargo workspace root (Cargo.toml) is not at the root of the derivation's source

Most cargo projects have their Cargo.toml at the root of the source, but it's still possible to build a project where the Cargo.toml file is nested in a deeper directory:

# Assuming that we have the following directory structure:
# ./flake.nix
# ./flake.lock
# ./nested
# ./nested/Cargo.toml
# ./nested/Cargo.lock
# ./nested/src/*.rs
craneLib.buildPackage {
 src = myLib.cleanCargoSource (craneLib.path ./.);
 cargoLock = ./nested/Cargo.lock;
 cargoToml = ./nested/Cargo.toml;
 # Use a postUnpack hook to jump into our nested directory. This will work
 # regardless of what the unpacked source is named (i.e. will avoid hashes
 # when using the root path of a flake).
 #
 # The unpackPhase sets `$sourceRoot` to the directory that was unpacked
 # but unfortunately `postUnpack` runs before the directory is actually
 # changed so we'll do two things:
 # 1. Jump into the directory we want (replace `nested` with your directory)
 # 2. Overwrite the variable so when the default build scripts run they don't
 # end up changing to a different directory again
 postUnpack = ''
   cd $sourceRoot/nested
   sourceRoot="."
 '';
}